In recent years it has become fashionable for high-end television receivers to include, in a menu, a feature in which a user may select one of several color temperature settings. Such a television receiver is the RCA CTC-170 color television receiver chassis, manufactured by Thomson Consumer Electronics, Inc., Indianapolis, Ind. The term "color temperature" refers to the degree of "whiteness" of white-colored areas of a picture displayed on a television display screen, relative to a given standard. Three standard light sources were chosen for measurement purposes; they are known as illuminant A, illuminant B, and illuminant C. Illuminant A is a tungsten filament incandescent lamp operating at 2500 degrees Kelvin. A white picture area which matches the particular shade of white light produced by this standard illumination source is said to have a color temperature of 2500 degrees. Illuminant B was selected to give an approximation of direct sunlight and was achieved by filtering the light from the tungsten lamp to produce a color equal to 4800 degrees Kelvin. Illuminant C filters the tungsten light to produce a color equal to 6500 degrees Kelvin, and is felt to produce a closer approximation of sunlight than Illuminant B. Illuminant C is the light source which was selected by the NTSC as a reference white for color television purposes. If the blue content of the white light is increased, the color temperature is raised, and the whites appear somewhat "whiter" to the human eye. If the red content of the white light is increased, the color temperature is lowered, and the whites appear somewhat "softer" to the human eye. Curiously, the perception of viewers is that as the color temperature is raised by adding a little more blue, the whites seem "cooler". Similarly, the perception of viewers is that as the color temperature is lowered by adding a little more red, the whites seem "warmer".
The NTSC color television system is based on the abovenoted specific color temperature and a specific set of very outdated picture tube phosphors. For example, a prior color matrix, produced by the former RCA Corporation, was designed for whites at 6500 degrees and exhibits a B-Y/R-Y ratio of 1.2. If a different color temperature, or different phosphors, are used, then color errors will exist. The color matrix can be designed to place these errors in the least conspicuous places. In recent years, the trend has been to increase color temperature in order to display "whiter" whites. For example, a known system is designed to display whites at 9300 degrees Kelvin and has a fixed B-Y/R-Y ratio of 1.1. To accomplish this, the color temperature is generally changed by altering the red or blue kine (i.e., picture tube) driver gain. This changes the ratio between red, green, and blue, and therefore the color temperature of white.
Unfortunately, in a television receiver in which color temperature is user-selectable, if the blue driver gain is increased to raise the color temperature, the magnitude of the B-Y color difference signal is also increased, which adversely alters the B-Y /R-Y ratio, causing colors near red to have too much blue content. and to appear magenta or purple. Similarly, if the red driver gain is reduced to raise the color temperature, the magnitude of the R-Y color difference signal is also reduced, which also adversely alters the B-Y/R-Y ratio, causing colors near red to have too much blue content, and to appear magenta or purple.